This invention relates to a gear drive or transmission. It relates especially to such a gear drive or transmission with a continuously variable input/output drive ratio.
1. Field of the Invention
A gear drive or transmission is often interposed between the motor and a driven device such as a rotor, shaft, wheel, etc. so that the device will rotate at a lower speed with higher torque than the motor shaft, or vice versa. A variable speed gear drive has a plurality of gears or gear sets which can be selectively interposed between the input and output shafts of the gear drive so as to change the gear or drive ratio of the drive.
2. Description of the Prior Art
There do exist in the prior art transmissions which have a continuously variable input/output drive ratio. Usually these have a first rotary member which is conical and a cylindrical rotary member, the two members being coupled together by an endless belt loop encircling the members. The torque applied to one member is coupled via the belt to the other member. The speed ratio may be changed by shifting the belt along the length of the conical member. In other words, if the conical member is rotated at a selected speed and the belt is located at the larger diameter end of the conical member, the other member will rotate at a relatively high speed. On the other hand, if the belt is located at the small diameter end of the conical member, the other member will rotate at a lower speed, the speed ratio being dependent upon the cone angle of the conical member.
Such transmissions employing belts are disadvantaged, however, in that there is slippage between the belt loop and the driving and driven members. Also the belt loop may stretch when under load. Consequently there is not a positive transmission of power between the driving and driven members.
In order to avoid the aforesaid problems, attempts have been made to construct transmissions or gear drives whose driving and driven members comprise gears whose teeth mesh to transmit power from the driving to the driven member. As shown in U.S. Pat. Nos. 5,608,390; 5,653,143 and 6,321,613, for example, the transmission may include a rotary conical gear member composed of a series of separate conical sections supported by rotary shaft and a second member in the form of a pinion slidably mounted to a second rotary shaft positioned alongside the conical gear member. The spur gear is rotatably coupled to the second shaft but slidable therealong so that the pinion can be positioned opposite any one of the conical sections making up the conical member so as to vary the input/output drive ratio of the transmission.
Such nominally continuously variable speed gear drives have a problem in that when the pinion is moved along its shaft to change the gear ratio of the transmission, it is momentarily disposed opposite two sections of the conical member at the same time. Since those sections have different diameters, they also have different numbers of teeth. Therefore, rather elaborate steps have to be taken to enable the pinion to mesh properly with the conical gear member at all positions of the pinion. Usually this involves providing a certain amount of rotary play between the various conical sections making up the conical gear member and coupling those sections to their common shaft by means of clutches. In other such drives, the conical gear sections making up the conical gear member are stepped along their diameters and provided with specially shaped teeth. Those attempted solutions devised to enable changing the drive ratio of such transmissions result in machines which are overly complex and costly. Furthermore, such transmissions do not really have a continuously variable drive ratio because the pinion cannot be left opposite two conical sections of the conical gear member at once for too long a time without causing excessive wear of the gear teeth and greatly increasing the likelihood that the transmission will freeze up or jam.
Accordingly, it is an object of the present invention to provide an improved variable ratio gear drive or transmission for transferring torque between an input shaft and an output shaft.
Another object of the invention is to provide a transmission of this type whose drive ratio is truly continuously variable over the entire operating range of the transmission.
Another object of the invention is to provide such a gear drive which is less complex than prior comparable variable ratio positive drive transmissions of this general type.
Other objects will, in part, be obvious and will, in part, appear hereinafter.
The invention accordingly comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the following detailed description, and the scope of the invention will be indicated in the claims.
Briefly, my transmission comprises a first rotary shaft which carries a continuous coaxial helical toothed rack whose diameter progressively increases along the shaft. Positioned parallel to the toothed surface of the rack as a second rotary shaft which carries a pinion whose teeth mesh with those of the rack. The pinion is rotatably coupled to its shaft but slidable therealong so that its teeth can mesh with those of the rack at any location along the length of the rack.
The drive ratio of the transmission may be changed by sliding the pinion along its shaft by hand or by other suitable means such as a linear actuator, lead screw drive, piston, etc. Either one of the two shafts may function as the driving or input member, the other shaft then being the driven or output member. In either event, since the slidable pinion may remain in driving engagement with the rack at any point along the length of the rack, the transmission does have a drive ratio which is truly continuously variable over the entire operating range of the transmission.
Since the helical rack is a single continuous member, it may be connected directly to its shaft without the imposition of clutches and other such devices that are found in prior gear drives whose conical gear members are composed of a series of separate conical gear sections.